Centrifuge decanters are well known. Such apparatus typically comprises an elongated bowl mounted for rotation about its longitudinal axis, with a helical screw conveyor coaxially mounted within the bowl, adapted to rotate at a speed slightly different than the rotational speed of the bowl. The bowl is tapered or trunco-conical near its solids discharge end. The screw conveyor is formed of one or more helically arranged blades which sweep the surface of the bowl of the apparatus while propelling the centrifugally sedimented solids toward the solids discharge port. The inner layer of light liquid is discharged from the liquid pool through overflow ports in the bowl end, opposite the solids discharge ports.
In operation of a centrifuge decanter with a screw conveyor, a solids-liquid feed is introduced into a middle portion of the bowl, where, due to centrifugal force effected by rotation of the bowl, the feed separates into its component parts with the heavier part, typically solids, being moved outward from the other feed components in a pool of liquid and adjacent to the inner surface of the bowl. Since the bowl and screw conveyor are rotated at predetermined different speeds, solids sedimented against the inner surface of the bowl are conveyed by the distal edge of the conveyor's blade along the bowl surface until separated from the pool of liquid and discharged from one or more ports at the tapered end of the bowl.
O'Conor in U.S. Pat. No. 3,423,015 described horizontal type continuous centrifugal separators including stationary pipes extending axially into the rotating element of the conveyor assembly for discharging feeds therein. Other horizontal centrifuge apparatus with extended stationary feed pipes are described, for example, in U.S. Pat. Nos. 3,228,594 to Amero; 3,447,742 to Eriksson et al.; 3,795,361 to Lee; 3,971,509 to Johnsen; 4,313,559 to Ostkamp et al.; 4,496,340 to Redeker et al.; 4,654,022 to Shapiro, 4,731,182 to High; and 5,182,020 to Grimwood.
Commercially important uses of centrifuge decanters include separation of solid crystalline chemical compounds from liquids under process conditions which do not degrade quality such as chemical purity of a desired crystalline product. Crystallization, as a commercial process, is significant because of the great variety of materials that are marketed in the crystalline form. Its wide use is due basically to the fact that a crystal forming from an impure solution is itself generally pure. Thus, crystallization affords a practical method of obtaining concentrated chemical substances in a form both pure and attractive, and in suitable condition for packaging, handling, and storing.
Solid particulate or crystalline products are handled and marketed more conveniently and economically than products in solution. Separations of a particulate solid or crystalline phase from a liquid phase by cooling, evaporation, or both, are well known. For example, separation of salt from sea water by solar-evaporation may be prehistoric.
Crystallization is also important in the preparation of a pure product since a crystal usually separates out as a substance of definite composition from a solution of varying composition. Impurities in the mother liquor are carried in the crystalline product only to the extent that they adhere to the surface or are occluded within crystals which may have grown together during crystallization.
Many organic compounds are formed by chemical reactions in a liquid phase, or are at least sparingly soluble in liquid solvents. To purify such compounds by means involving treating solutions of them and/or recovering solid product from a liquid phase requires some means of crystallization. Aromatic dicarboxylic acids are, for example, well known starting materials for making polyester resins, which polyester resins are used widely as principal polymers for polyester fibers, polyester films, and resins for bottles and like containers. For a polyester resin to have properties required in many of these uses, the polyester resin must be made from a polymer grade or "purified" aromatic acid. Polymer grade or purified terephthalic acid and isophthalic acid are the starting materials for polyethylene terephthalates and isophthalates, respectively, which are the principal polymers employed in the manufacture of polyester fibers, polyester films, and resins for bottles and like containers. Similarly, polymer grade or "purified" naphthalene dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid, are the starting materials for polyethylene naphthalates, which can also be employed in the manufacture of fibers, films and resins.
Commonly assigned U.S. Pat. No. 3,497,552 to Olsen discloses that purification of impure organic compounds sparingly soluble in a liquid such as water can be accomplished by continuous crystallization in a plurality of series-connected cooling stages using dilutions by cooled solvent of feed at each stage.
A purified terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid can be derived from a relatively less pure, technical grade or "crude" terephthalic acid, isophthalic acid or "crude" naphthalene dicarboxylic acid, respectively, by purification of the crude acid utilizing hydrogen and a noble metal catalyst as described for terephthalic acid in commonly assigned U.S. Pat. No. 3,584,039 to Meyer. In the purification process of Meyer, impure terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid is dissolved in water or other suitable solvent or solvent mixture at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, which conventionally is palladium on a carbon support, as described in commonly assigned U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step converts the various color bodies present in the relatively impure phthalic acid or naphthalene dicarboxylic acid to colorless products. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in commonly assigned U.S. Pat. No. 4,405,809 to Stech et al.
Aromatic carboxylic acids are useful chemical compounds and are raw materials for a wide variety of manufactured articles. For example, terephthalic acid is manufactured on a world-wide basis in amounts exceeding 10 billion pounds per year. A single manufacturing plant can produce 100,000 to more than 750,000 metric tons of terephthalic acid per year. Terephthalic acid is used, for example, to prepare polyethylene terephthalate, a raw material for manufacturing polyester fibers for textile applications and polyester film for packaging and container applications. Terephthalic acid can be produced by the high pressure, exothermic oxidation of a suitable aromatic feedstock compound, such as para-xylene, in a liquid-phase reaction using air or other source of dioxygen (molecular oxygen) as the oxidant and catalyzed by one or more heavy metal compounds and one or more promoter compounds.
Methods for oxidizing para-xylene and other aromatic compounds using such liquid-phase oxidations are well known in the art. For example, Saffer in U.S. Pat. No. 2,833,816 discloses a method for oxidizing aromatic feedstock compounds to their corresponding aromatic carboxylic acids. Other processes are disclosed in U.S. Pat. Nos. 3,870,754; 4,933,491; 4,950,786; and 5,292,934. A particularly preferred method for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid is disclosed in U.S. Pat. No. 5,183,933. Central to these processes for preparing aromatic carboxylic acids is employment of an oxidation catalyst comprising a heavy metal component and a source of bromine in a liquid-phase reaction mixture, including a low molecular weight monocarboxylic acid such as acetic acid, as part of the reaction solvent. A certain amount of water is also present in the oxidation reaction solvent, and water is also formed as a result of the oxidation reaction.
Although petroleum, more particularly reformate fractions produced in petroleum refining, provides a valuable source of para-xylene, separation of the para-xylene from associated, close boiling hydrocarbons presents a difficult commercial problem. Some areas of the chemical market require a para-xylene of at least 98% purity, which means it cannot be recovered by fractional distillation or simple crystallization in reasonable enough yield for economic feasibility. There have been a number of approaches to the problem, using fractional crystallization, for example, but the high cost of the available processes in terms of equipment and operational expense makes further simplification highly desirable. Any improvement in ultimate yield improves the economic attractiveness of the process and reduces the unit cost.
Methods for separation of para-xylene from other aromatic compounds by crystallization are well known in the art. For example, G. C. Lammers in U.S. Pat. No. 3,177,265 discloses a particularly efficient method for recovering para-xylene by crystallization from a C8 or mixed xylene feed which utilizes a two-stage crystallization process with centrifugal separation following each stage. By use of this method 98+% para-xylene product is obtainable. It has been further found that stepwise cooling in the first stage facilitates crystal growth which enhances ease of separation of the mother liquor from crystal cake. Other processes are disclosed in U.S. Pat. Nos. 3,462,509 to Dresser et al.; 3,720,647 to Gleb et al.; 3,723,558 to Kramer; 4,721,825 to Oda et al.; 5,448,005 and 5,498,822 to Eccli et al.
Extraction of high purity para-xylene crystals from a feed of mixed xylenes and impurities has included the steps of cooling a feed of mixed xylenes and impurities in a crystallization stage to crystallize out para-xylene, separating the liquid component comprising ortho-xylene and meta-xylene and impurities from the solid crystal para-xylene in a centrifuge to obtain high purity para-xylene, supplying the mixed liquid (xylenes and impurities) filtrate, including para-xylene melted due to centrifuge work input and heat from the environment, to a holding drum, supplying the all liquid filtrate to an isomerization stage where the filtrate is reacted over a catalyst bed, separating para-xylene and mixed xylenes from impurities in a distillation stage and recycling the mixed xylenes to the crystallization stage.
Commonly assigned U.S. Pat. No. 5,004,860 issued Aug. 2, 1991 to John S. Hansen and William A. Waranius discloses a filter system which is coupled to a crystallizer in a liquid crystal separation unit and a method for using the same for extracting liquid from a liquid crystal slurry to enhance solid crystal recovery. More specifically, the patent relates to a filter system comprising porous metal tubes having very small porosity which are utilized in a closed feedback loop of liquid-crystal slurry for extracting liquid filtrate from the slurry and returning the higher crystal concentration slurry back to a crystallizer in a process for the extraction of para-xylene crystals from a mother liquor feed including mixed xylenes and impurities in liquid and crystal form.
Because it is usually preferred that fairly dry solids and clear liquid be separately discharged from opposite ends of the centrifuge bowl, the solids-liquid feed must be introduced into the pool of liquid at the middle portion of the bowl rather than near either end. Therefore, the solids-liquid feed is usually delivered into the conveyor from the distal end of a small stationary feed tube extended into the centrifuge along its rotational axis. Problems which persist with an extended stationary feed tube supported only near one end of the bowl include deflections and vibration of the cantilevered tube.
Due to changes in solids content of the feed during operation, variations in the weight of feed loading an extended stationary feed tube can cause significant deflections and vibrations of the tube and rotating parts of the centrifuge. Typically, even brief contact between, for example, the distal end of a small stationary tube and any rotating parts of the centrifuge is likely to be catastrophic to an extended stationary feed tube.
Solid deposits from the feed can form incrustations on an extended stationary feed tube to cause significant deflections, vibrations, and can even cause an avalanche of solids onto the conveyor or into the liquid pool which are rotating at high speeds. Such an avalanche is likely to cause a rapid increase in power required to drive the centrifuge which may subsequently be taken out of service for mechanical inspection and/or maintenance.
There remains, therefore, a current need for centrifugal apparatus which provides improved means for feeding solids-liquid mixtures which is effective in reducing the magnitude of mechanical vibration, reducing feed tube failures, and thereby avoiding interruptions in service.
Advantageously, such improved means of feeding solids-liquid mixtures would assist in acceleration of the mixtures up to rotating speed with decreased damage of the solid crystals, improving their recovery.